1. Field of the Invention
The field of the invention is electrical switches, and more particularly, transparent membraneous switches known as touch panel switches or touch screen switches.
2. Description of the Background Art
Transparent touch screens are used as input devices for computers, often being disposed over the screen of a monitor or CRT or other type of visual display. Two types of resistive touch screen switches are "analog resistive" and "matrix". In an analog resistive touch screen, the location of the touch is decoded by analyzing the screen as a voltage divider in the X-direction and in the Y-direction based on voltage readings in the X-direction and Y-direction, respectively, caused by a touch anywhere on the screen. In matrix switches, the contacts on one layer are conductive strips running in an X-direction and opposing contacts on a second layer are conductive strips running in a Y-direction, so that each switch location is defined by the intersection of an X-direction conductive strip and a Y-direction conductive strip.
Both analog resistive and matrix touch screens are electrical contact devices with resistance type contacts. Some of these devices utilize switch contacts and switch conductors formed of indium tin oxide (ITO) or tin oxide, which are semiconductive ceramic materials exhibiting transparency and light transmission qualities which are advantageous for application to touch screens.
When resistive touch screens are operated, contact is made between opposing surfaces of ITO or tin oxide. Electrical contact resistance has been observed to increase significantly after many cycles of operation (switch closures). This can cause problems with switch reliability.
When the switch contacts are closed, a very small amount of localized surface deterioration takes place. If the switch is closed many times in one location, this deterioration may cause an increase in contact resistance over time. If the contact resistance between the two conductive planes of thin film becomes large enough to no longer be considered insignificant, the decoding circuitry can no longer determine the position of the touch, which will eventually lead to switch malfunction.
There is a problem of increasing contact resistance over the life of resistive touch screens. The life of a touch screen is one of its more important characteristics. One commercial objective is that a touch screen should last as long as the display on which it is used. Improvement in maintaining contact resistance improves the important performance areas of product life and switch function consistency.